1. Field of the Invention
The invention relates to organic compounds having a core-shell structure, to a process for preparing them and to their use as semiconductors in electronic components.
2. Brief Description of the Prior Art
The field of molecular electronics has developed rapidly in the last 15 years with the discovery of organic conductive and semiconductive compounds. In this time, many compounds which have semiconductive or electrooptical properties have been found. While it is generally accepted that molecular electronics will not displace conventional semiconductor building blocks based on silicon, it is believed that molecular electronic components will open up new applications in which suitability for coating large areas, structural flexibility, processability at low temperatures and low costs are required. Semiconductive organic compounds are at present being developed for applications such as organic field effect transistors (OFETs), organic luminescence diodes (OLEDs), sensors and photovoltaic elements. Simple structuring and integration of OFETs into integrated organic semiconductor circuits make it possible to achieve inexpensive solutions for smart cards or price signs which have hitherto not been realized with the aid of silicon technology, owing to the price and the lack of flexibility of the silicon building blocks. Likewise, OFETs can be used as circuit elements in large-area flexible matrix displays. An overview of organic semiconductors, integrated semiconductor circuits and their uses is given, for example, in Electronics 2002, volume 15, p. 38. A field effect transistor (FET) is a three-electrode element in which the conductivity of a thin conduction channel between two electrodes (known as “source” and “drain”) is controlled by means of a third electrode (known as the “gate”) separated from the conduction channel by a thin insulating layer. The most important characteristic properties of a field effect transistor are the mobility of the charge carriers, which has a critical effect on the switching speed of the transistor, and the ratio of the currents in the switched-on and switched-off state, known as the “on/off ratio”. Two large classes of compounds have hitherto been used in organic field effect transistors. All these compounds have long conjugated units and are classified into conjugated polymers and conjugated oligomers on the basis of the molecular weight and structure. Oligomers generally have a uniform molecular structure and a molecular weight of less than 10 000 dalton. Polymers generally consist of chains made up of uniform repeating units and having a molecular weight distribution. However, there is a continuous transition between oligomers and polymers. The distinction between oligomers and polymers is frequently made to reflect the fact that there is a fundamental difference in the processing of these compounds. Oligomers are frequently vaporizable and are applied to substrates by vapour deposition processes. The term polymers is frequently used to refer to compounds which are no longer vaporizable and are therefore applied by other methods, regardless of their molecular structure. It is generally desirable for polymers to be soluble in a liquid medium, for example organic solvents, and then be able to be applied by corresponding application methods. A very widespread application method is, for example, spin coating.
A particular elegant method is the application of semiconductive compounds by means of the inkjet process. In this process, a semiconductive solution is applied in the form of very fine droplets to the substrate and is dried. This method allows structuring to be carried out during application. A description of this method of applying semiconductive compounds is given, for example, in Nature, volume 401, p. 685. In general, the wet chemical methods are considered to have a greater potential for obtaining inexpensive integrated organic semiconductor circuits in a simple way.
An important prerequisite for the production of high-quality organic semiconductor circuits is compounds of extremely high purity. In semiconductors, ordering phenomena play an important role. Organic semiconductor circuits which have been constructed using compounds which are not of extremely high purity are generally unusable because they can hinder uniform alignment of the compounds. Hindering of a uniform alignment of the compounds and pronounced grain boundaries leads to a dramatic drop in the semiconductor properties. Also, residual impurities can, for example, inject charges into the semiconductive compound (“doping”) and thus reduce the on/off ratio or act as charge scavengers and thus drastically reduce the mobility. Furthermore, impurities can initiate the reaction of the semiconductive compounds with oxygen and impurities having an oxidizing action can oxidize the semiconductive compounds and thus shorten possible storage, processing and operating lives.
The purities which are generally necessary are generally not achievable by known methods of polymer chemistry, e.g. washing, reprecipitation and extraction. On the other hand, oligomers as molecularly uniform and frequently volatile compounds can be purified relatively easily by sublimation or chromatography.
Some important representatives of semiconductive polymers are described below. Polyfluorenes and fluorene copolmers, for example poly(9,9-dioctylfluorene-co-bithiophene) (I)
have been able to achieve charge mobilities, hereinafter also referred to as mobilities for the sake of brevity, of up to 0.02 cm2/Vs (Science, 2000, volume 290, p. 2123), and regioregular poly(3-hexylthiophene-2,5-diyl) (II)
has even been able to achieve mobilities of up to 0.1 cm2/Vs (Science, 1998, volume 280, p. 1741). Polyfluorene, polyfluorene copolymers and poly(3-hexyl-thiophene-2,5-diyl), like virtually all long-chain polymers, form good films after application from solution and are therefore easy to process. However, as high molecular weight polymers having a molecular weight distribution, they cannot be purified by vacuum sublimation and only with difficulty by chromatography.
Important representatives of oligomeric semiconductive compounds are, for example, oligothiophenes, in particular those having terminal alkyl substituents as represented by the formula (III),
and pentacene (IV)

Typical mobilities for, for example, α,α′-dihexyl-quarterthiophene, -quinquethiophene and -sexithiophene are 0.05–0.1 cm2/Vs.
Mesophases, in particular liquid-crystalline phases, appear to play an important role in semiconductive organic compounds, but this has hitherto not been fully understood by experts in the field. For example, the highest mobility has hitherto been reported for crystals of α,α′-dihexylquarterthiophenes (Chem. Mater., 1998, volume 10, p. 457), with these crystals crystallizing from an enantiotropic liquid-crystalline phase at a temperature of 80° C. (Synth. Met., 1999, volume 101, p. 544). Particularly high mobilities can be obtained when using single crystals, e.g. a mobility of 1.1 cm2/Vs has been described for single crystals of α,α′-sexithiophenes (Science, 2000, volume 290, p. 963). If oligomers are applied from solution, the mobilities usually drop considerably.
In general, the deterioration in the semiconductive properties when oligomeric compounds are processed from solution is attributed to the moderate solubility and low tendency to form films of the oligomeric compounds. Thus, inhomogeneities are attributed, for example, to precipitates formed from the solution during drying (Chem. Mater., 1998, volume 10, p. 633).
Attempts have therefore been made to combine the good processing and film-forming properties of semiconductive polymers with the properties of semiconductive oligomers. U.S. Pat. No. 6,025,462 describes conductive polymers having a star structure consisting of a branched core and a shell of conjugated side groups. However, these polymers have a number of disadvantages. If the side groups are formed by laterally unsubstituted conjugated structures, the resulting compounds are sparingly soluble or insoluble and cannot be processed. If the conjugated units are substituted by side groups, this does lead to an improved solubility but the side groups cause internal disorder and morphological disruptions as a result of the space they take up, resulting in impairment of the semiconductive properties of these compounds.
WO 02/26859 describes polymers having a conjugated backbone to which aromatic conjugated chains are attached. The polymers bear diarylamine side groups which make electron conduction possible. However, the diarylamine side groups make these compounds unsuitable as semiconductors.
There is therefore a continuing need for compounds which combine the properties of organic semiconductive oligomers and polymers.
It is therefore an object of the invention to provide organic compounds which can be processed from customary solvents and have good semiconductive properties. Such organic semiconductive compounds would be extremely suitable for coating large areas.
It would be desirable for the compounds to form high-quality layers of uniform thickness and morphology and to be suitable for electronic applications.